External stair - means of escape

[massive1969]

Sorry - silly question time...

This is a specific case; but is also a general question as to whether internal stairs must be protected if there is also a second means of escape being an external stair.

An existing building which is being converted from a residential accommodation (small block of flats) in to an office/admin block. There is ample room for an external stair to be fitted.

Built 2008.

Three storeys (ground, first and second) plus basement/cellar. 60-min fire separation between basement/cellar and ground floor.

Footprint of each floor is approximately 10m x 12m.

L3 alarm system / emergency escape lighting being installed.

Risers will be enclosed within 30-min protection.

Standard construction (brick, breeze block, concrete floors, flat roof).

Occupancy levels will be approximately 10 per floor (none in basement/cellar which is used for plant room / electrics).

The client wants a complete open plan design with an open internal staircase. Building Control will be involved - client is at drawings/plans stage at present.

The glazing along the external stair with 30-minute glass which is sealed closed.

The fire door from first floor leading to external stair will also be FD30 standard/

Question: I just want to double check; is the open plan internal stair acceptable as long as;

A) External escape stair is fully protected along route
B) Travel distances from any point in the first & second floor levels are acceptable (less than 18m in single direction to only protected means of escape)
C) Occupancy levels from each floor are less than 60

I've been reading BS9999 & ADB which I think is relevant as this is a complete conversion of a building. Item 18.7 of BS9999 in particular.

Or am I misunderstanding this and does the internal stair also have to be protected??

In view of low occupancy levels; I think that only 1 means of escape from each floor is acceptable as long as the travel distances are within 18m?

In your experience(s), are there any other compensatory features the client could consider which may make an open internal stair acceptable?? Higher levels of AFD perhaps? Sprinklers??

Any help or guidance in right direction much appreciated!! Sorry - new query for me!

[Golden]

Could I just ask what your role is in this building?

[massive1969]

Hi Golden,

I am a trainee at a fire safety consultant - we have been given this case to help a client with and I am shadowing a consultant but wanted to try and do some background work on this before we sat down and went through it in detail.

I've not come across a case where the client wants a completely open plan staircase and wondered if this was going to be acceptable.

More of a general query really, but provided some specifics to help.

[Dinnertime Dave]

Unusual, but I can't see anything that stops this approach.

If there is a lift then persons with disabilities will need a refuge, that can be on an external stair but might be uncomfortable at this time of the year.

Ground and first floor doors to external need to be FD30S and top floor as well if you putting a disabled refuge outside.

[massive1969]

Thanks Dave.  There is no lift mentioned in plans we have.

[AnthonyB]

Our offices, which were a new build, followed a very similar design. Single internal stair, serving ground, first and second floors, all open (almost like one big atrium with two mezzanines) and an external stair at the opposite end with the usual protective walls and doors, to the open stair serving the 1st & 2nd floors with lobby protection.

The only BCO requirement was instead of the default Category M system for an office it had to be L1 and with the internal stair being classed as an accommodation stair travel distances to the protected lobby had to meet single direction of escape limits.

[Dinnertime Dave]

The only BCO requirement was instead of the default Category M system for an office it had to be L1

Wouldn't be much difference between L1 and L3 in these circumstances.

[massive1969]

Thats a great help - thank you guys.

To change it from a L3 to L1 would be minimal - just a couple of rooms / stores on each floor so we may as well go for the L1 option to make sure its agreed by the BCO.

Thanks again - very much appreciated.